The present invention relates to the field of air bearings, and more particularly to an air bearing slide assembly where an outer carriage assembly is slidably supported over an inner ram by a plurality of opposed air bearings contained within the carriage. The air bearings allow for precision linear movement of the carriage relative to the ram.
There has long been a need in the field of precision machining and other areas for linear slides capable of precise, repeatable, vibration free motion. Such linear slides have been primarily of two types. In the first type, a movable carriage traverses a fixed ram (essentially a beam) and in the second type, a movable ram slides within a fixed housing. Of the two types, in the latter arrangement, where the ram moves relative to a fixed housing, the ram may be extended a substantial distance from the frame. This cantilevered extension of the movable ram may cause the ram to droop from the housing or otherwise displace from a desired longitudinal axis. Such displacement creates undesirable vertical position error at the extended end of the ram. In the former arrangement, where the ram is fixed at both ends with the carriage traversing the ram, vertical deflection is minimized. For this reason the fixed ramxe2x80x94movable carriage type of linear slide is preferred.
In the past, contact bearings such as ball, roller, or tapered roller bearings were used to facilitate movement of the carriage along the ram. Contact bearings are relatively inexpensive and have high load carrying capability. Contact bearings remain in widespread use where high precision positioning is not required or where highs loads are involved.
More recently, hydrostatic bearings, more commonly referred to as air bearings, have been incorporated into linear slides for use in applications where extremely precise, smooth, repeatable motion, on the order of a few micro-inches, is required. Air bearings make use of well know hydrostatic principles. In an air bearing, a flat sliding surface in the carriage rests against a flat sliding surface on the ram. The air bearing operates by supplying pressurized air or other gas to an inlet orifice where the orifice acts as a throttle to produce a desired volume flow rate. After throttling, the air is ejected outwardly from the bearing. The outwardly ejected air produces a high pressure air cushion upon which the carriage rides over the ram. Air bearing equipped linear slides have minimal static friction and therefore offer the capability for increased smoothness and enhanced positioning accuracy, as well as a greater degree of vibration isolation than can be achieved in a contact bearing equipped slide.
Despite their advantages over contact bearing equipped slides, the use of air bearing equipped slides is limited by their high cost. For air bearings to be useful for precise positioning, high repeatability of position is desired for the movable carriage. Such repeatability is dependant not only on the means for moving the carriage (e.g., stepper motor, belt drive mechanism, etc.), but also on the precise mating of the air bearing surface with that of the ram.
Typical prior art air bearing equipped linear positioning slides are manufactured by New Way Machine Components, 4009 Market Street, Aston, Pa., and Dover Instrument Corporation, P.O. Box 200, Westboro, Md. These slides typically utilize a fixed ramxe2x80x94movable carriage design utilizing from 4-6 circular air bearings on the carriage sliding surfaces which are mated to the ram. Multiple bearings per slide surface are generally required to provide the linear slide assembly with a load carrying capacity sufficient to be useful in precision machining and other applications.
One problem common to most prior art air slides is alignment of the bearings. Typically, prior art slides mount each air bearing in a discrete bearing holder which is then installed in a bearing carrier, which is subsequently integrated into the carriage assembly. To provide for effective slide operation, each air bearing on a particular carriage slide surface must be aligned to be substantially coplanar with every other bearing on that surface. Since air bearing to ram slide surface air gaps are typically on the order of 5 to 20 microns, bearing alignment is both a critical and expensive process. Generally, alignment costs increase proportionately with the number of bearings per individual slide surface.
Air slides with misaligned bearings are susceptible to self-excited vibration or xe2x80x9cwhirringxe2x80x9d of the carriage assembly. Typically, non-coplanar bearings generate eddy currents in the air cushion between the carriage and the ram. These eddy currents tend to induce vibration or whirring in the carriage as the carriage rides over the ram. Whirring significantly degrades carriage positioning accuracy. The problem of self-excited vibration may be compounded by the prior art methods of moving or driving the carriage. Prior art carriage drives are typically linear motors, ball and screw leads, and belt drive mechanisms. The vibration caused by these drive mechanisms may exacerbate xe2x80x9cwhirringxe2x80x9d in a linear air slide.
An additional factor which tends to increase the cost of an air slide is the use of individual air supply hoses for each bearing. Although effective, this method of supplying pressurized air leads to a multiplicity of hoses and fittings which further complicates the air slide.
What is needed therefore is an air slide design that is substantially less costly to produce. Such a design would use a minimum of air bearings on each slide surface thereby reducing the difficulties associated with bearing alignment, yet would still provide at least as much load carrying capacity as the prior art slides. Such a design would also incorporate air feed passages internally within the carriage assembly to eliminate the multiplicity of air supply lines as is found in the prior art. Such a design would further reduce the number of component parts by mounting the air bearings directly within the carriage assembly and would use interchangeable modular components to further reduce complexity and costs. Finally, such a design would incorporate a more precise drive motor or actuating device for positioning the carriage assembly.
The present invention is an air bearing slide assembly for controlled linear motion. The slide assembly of the present invention overcomes many of the disadvantages of prior art air bearing slides. More precise position location is possible with the present slide assembly. It is more versatile than prior art linear slides in function and design, and is of quite simple manufacture, while achieving greater precision in bearing alignment and mating of the relative sliding surfaces.
The air bearing slide assembly of the present invention features a simplified carriage assembly including a first set of longitudinally extending upper and lower bearing carriers, where the upper and lower carriers are preferably interchangeable. The carriage assembly also includes a second set of left and right longitudinally extending bearing carriers, which are also preferably interchangeable. The four bearing carriers are integrated to form the carriage assembly. When joined, the carriers combine to form a longitudinal throughbore for mating with a slide ram.
In an exemplary embodiment, the upper and lower bearing carriers each mount two elongated planar rectangular air bearings, while the left and right bearing carriers each mount one elongated planar rectangular air bearing. By using a minimum number of bearings, the bearing alignment process is simplified. Further, rectangular air bearings have a greater load carrying capacity for a given width than do circular bearings of a similar diameter. To minimize the number of component parts, the air bearings in the present invention slide are preferably mounted directly in the bearing carriers. In addition, each carrier is preferably formed with internal air passages which interface with each mating bearing carrier so that a single external air supply port may pressurize all of the bearings in the carriage assembly.
The air bearing slide assembly of the present invention also includes an elongated slide ram which has a central longitudinal axis. The exterior of the slide ram is generally defined by a plurality of symmetrically opposed planar and longitudinally extending ram slide surfaces.
The slide ram is slideably receivable in the carriage assembly throughbore with the ram slide surfaces and the air bearing slide surfaces being in closely spaced slidable mated alignment to permit longitudinal movement of the carriage assembly relative to the slide ram. Precise mating of the air bearing slide surfaces to the ram slide surfaces is achieved by individual lapping of the mating surfaces. Such lapping is done to such an extent that an air bearing to ram air gap of typically about 5 microns to 20 microns is achieved. In operation, pressurized gas, typically compressed air, is supplied to the air bearings so that hydrostatic fluid support films, or air cushions, are created between the air bearings and the ram slide surfaces. Thereby, the carriage is suspended on the ram and is free to move longitudinally over the ram.
The present invention air slide also includes a piezo-electric drive motor which may be incorporated entirely within the carriage assembly and operably coupled to an indexed drive strip on the ram. The piezo-electric drive motor provides for more accurate positioning and smoother operation than has heretofore been possible. The air slide may further include a digital encoder which is also incorporated entirely within the carriage assembly and operably coupled to an indexed drive strip on the ram. Other features and advantages of the invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.